1. Field of the Invention
The present invention relates to a method and an apparatus for the establishing of the surface tension of the interface between a gas and a liquid medium, and more particularly to a method and apparatus for determining the surface tension of a pulmonary surfactant.
2. Description of the Prior Art
In the past several decades, there has developed a greatly increased understanding of the surfactant system of the lung. Monolayers of lung alveolar surfactant have been shown to develop near zero surface tensions upon dynamic compression in known types of apparatus. The common characteristics of the studies, especially if they are done at 37.degree. C., have involved the requirement of a minimum film compression rate for the surface balance technique or a minimum cycling speed for the pulsating bubble technique. See Goerke, J. and Clements, J. A. (1986); Alveolar surface tension and lung surfactant; In: Handbook of Physiology, Section 3, The Respiratory System. Vol. 111 part 1, edited by A. P. Fishman, P. T. Macklem, J. Mead and S. R. Geiger. Bethesda, Amer. Physiol. Soc. pp 247-261. Although minimum surface tensions below 5 mN.m.sup.-1 in the film balance or below 1 mN.m.sup.-1 for the pulsating bubble, can be obtained if the compression rate for pulmonary surfactant films is fast enough, these low surface tensions obtained in vitro have much less stability than the alveolar film in situ.
The stability of the alveolar film in situ has been calculated from lung pressure-volume studies of animal lungs. In lungs held at 40% total lung capacity, the surface tension increased by only 1 to 2 mN.m.sup.-1 in 20 minutes. See Horie, T. and Hildebrandt, J. (1971); Dynamic compliance, limit cycles, and static equilibria of excised cat lungs. J. Appl. Physiol. 31:423-430. A similar surface tension stability has been obtained by observing the spreading behaviour of fluid droplets placed by micropipets into individual alveolar spaces. See Schurch, S. (1982); Surface tension at low lung volumes: dependence on time and alveolar size. Respir. Physiol. 48:339-355. According to these studies, there is no need for a minimum compression rate of the alveolar film to obtain a near zero and stable surface tension. One inflation-deflation cycle may take between 10 and 30 minutes.
In surface balance experiments, one compression-expansion cycle usually takes 1 to 2 minutes and if the barrier movement is stopped at minimum surface tension, this surface tension may increase in a few minutes to more than 10 mN.m.sup.-1. For the pulsating bubble method between 10 and 20 cycles per minute are used in order to produce near zero surface tension. However, if cycling is stopped at minimum surface tension, the surface tension increases even more rapidly than in the film balance technique. The reason for this in vitro instability is the relatively large escape route for the film molecules as will be described in more detail below. In the pulsating bubble system, the bubble, which is between 0.8 and 1.1 mm in diameter, is connected to atmospheric air via a plastic tubing of about 0.8 mm in diameter. Only relatively fast cycling can compensate for the film loss up the tubing.
To further describe the above-mentioned bubble system, reference is made to a series of articles by R. E. Pattle, including an article entitled "Maturity of Fetal Lungs Tested by Production of Stable Microbubbles in Amniotic Fluid", British Journal of Obstetrics and Gynecology, August, 1979, Vol. 86, pp 615-622 and "Properties, function, and origin of the alveolar lining layer" (1957). Pattle describes that lung tissue contains surfactant material that stabilize microscopic bubbles suspended in water. He observed such bubbles suspended in a drop of water under a microscope slide. Pattle realized that bubbles trapped under the glass slide contracted and flattened because of the relatively rapid diffusion if he used deaerated water as the suspending medium for the bubbles. It was concluded from the life time of these bubbles that the surface tension of lung extract films must approach zero. Pattle apparently did not realize that use could be made of shape analysis of the deformed bubbles to obtain the surface tension accurately and the corresponding bubble surface areas to construct surface tension vs. area relations.
In a publication entitled "Pulsating bubble technique for evaluating pulmonary surfactant", by Goran Enhorning, there is described a technique of determining surface tension with an apparatus that records pressure across the surface of a bubble, expanded in a sample liquid and communicating with ambient air. Pressure pulsations are described as causing the bubble to expand and contract periodically. From the known pressure gradient across the bubble surface and the bubble radius, the surface tension can be calculated with the law of Laplace. The bubble communicates with the ambient air via a capillary tube, and because the ratio of bubble radius to the tube diameter is relatively small, the surfacted film material has a large escape route. In order to reach near zero surface tension, the bubble has to be compressed at a certain minimum compression rate in order to compensate for the escape of film materials through the tubing. Thus, the method is only suitable for dynamic surface tension measurements because of the film material leakage up the tubing.
In a published articles entitled, "Surface Tension at Low Lung Volumes: Dependence on Time and Alveolar Size" by Samuel Schurch, Respiratory Physiology (1982) 48, 339-335, Elsevier Biomedical Press, "New Insights into Principles of Function and Physiology of Surfactant, Including its Role in Small Airways", by Samuel Schurch, progress in respiratory research, volume 18, pp. 1-9 (1984), there is described technology relating to large captive bubbles whose volumes are controlled by introducing and withdrawing air. The surface tension and bubble areas are determined by shape analysis using established computer approximations. Although the ratio of bubble surface area to tubing diameter is much greater than in the above described design of Enhorning's, there is still film material leakage through air capillary tubing which is used to change the bubble size.
Another described surface tension measuring technique is known as the Langmuir Wilhelmy method, and there are commercially available Langmuir film balance machines sold under the trade mark LAUDA by Sybron/Brinkmann.
The original Langmuir design is described in a text entitled "The Physics and Chemistry of Surfaces" by Neil Kensington Adam, Dover 1968.
The original Langmuir Wilhelmy method was modified by John A. Clements as described in a publication by the Society for Experimental Biology and Medicine, entitled, "Surface Tension and Lung Extracts" by John A. Clements (1957); and a published article entitled "Surface Phenomena in Relation to Pulmonary Function", by John A. Clements, The Physiologist, Vol. 5, 1962; and in a publication entitled "Surface Active Materials from Dog Lung. I. Method of Isolation" by Richard J. King, and John A. Clements, American Journal for Physiology, volume 225 No. 3, September, 1972.
As described in a publication "Time Dependent Changes of the Surface Pressure of Alveolar Surface Layers in the Langmuir Trough" by W. Schoedel et al, Pflugers Arc. 306, pp 20-32 (1969) there is described the use of a rhombic frame to change the film area as a variation of the rectangular trough design of Clements.
A publication entitled "A New Surface Balance for Dynamic Surface Tension Studies" by Boyle and Mautone, Colloids and Surfaces, 4(1982) 77-85, there is described yet another variation of the Langmuir Wilhelmy design.
The above described designs are usable for static and dynamic surface tension vs. area studies.
The Langmuir Wilhelmy method is the one most commonly used in commercially available products. The above described apparatus sold under the trade mark "LAUDA" is a relatively large and expensive machine and is not portable. In this type of machine, there is provided a trough in which the film pressure is measured by a polytetrafluroethylene coated floating barrier. The nature of the operation of the machine necessitates the use of highly trained personnel. The machine, like the apparatus using the pulsating bubble method exhibits in vitro instability which is due to the relatively large escape route for the film molecules. In the surface balance machine, the molecules move up the barrier or escape around the polytetrafluroethylene barrier since the contact surface of the barrier and the water interface is well suited for surfactant spreading.
In the pulsating bubble method, between 10 and 20 cycles per minute are used in order to produce near zero surface tension.
However, if cycling is stopped at minimum surface tension, the surface tension increases even more rapidly than in the Langmuir surface balance apparatus. In the pulsating bubble system, the bubble, between 0.8 and 1.1 mm in diameter, is connected to the atmospheric air via a plastic tubing of about 0.8 mm in diameter. Only relatively fast cycling can compensate for the film loss up the tubing.